Hypoxia increases the activity of Ca2+-sensitive K+ channels in cat cerebral arterial muscle cell membranes

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Abstract
The cellular mechanisms mediating hypoxia-induced dilation of cerebral arteries have remained unknown, but may involve modulation of membrane ionic channels. The present study was designed to determine the effect of reduced partial pressure of O2, PO 2, on the predominant K+ channel type recorded in cat cerebral arterial muscle cells, and on the diameter of pressurized cat cerebral arteries. A K+-selective single-channel current with a unitary slope conductance of 215 pS was recorded from excised inside-out patches of cat cerebral arterial muscle cells using symmetrical KCl (145 mM) solution. The open state probability (NP o) of this channel displayed a strong voltage dependence, was not affected by varying intracellular ATP concentration [(ATP]i) between 0 and 100 μM, but was significantly increased upon elevation of intracellular free Ca2+ concentration ([Ca2+]i). Low concentrations of external tetraethylammonium (0.1–3 mM) produced a concentration-dependent reduction of the unitary current amplitude of this channel. In cell-attached patches, where the resting membrane potential was set to zero with a high KCl solution, reduction of O2 from 21% to < 2% reversibly increased the NP o, mean open time, and event frequency of the Ca2+-sensitive, high-conductance single-channel K+ current recorded at a patch potential of + 20 mV. A similar reduction in PO2 also produced a transient increase in the activity of the 215-pS K+ channel measured in excised inside-out patches bathed in symmetrical 145 mM KCl, an effect which was diminished, or not seen, during a second application of hypoxic superfusion. Hypoxia had no effect on [Ca2+]i or intracellular pH (pHi) of cat cerebral arterial muscle cells, as measured using Ca2+- or pH-sensitive fluorescent probes. Reduced PO2 caused a significant dilation of pressurized cerebral arterial segments, which was attenuated by pre-treatment with 1 mM tetraethylammonium. These results suggest that reduced PO2 increases the activity of a high-conductance, Ca2+-sensitive K+ channel in cat cerebral arterial muscle cells, and that these effects are mediated by cytosolic events independent of changes in [Ca2+]i and pHi.